Drilling and producing offshore oil and gas wells includes the use of offshore platforms for the exploitation of undersea petroleum and natural gas deposits. In deep water applications, floating platforms (such as spars, tension leg platforms, extended draft platforms, and semi-submersible platforms) are typically used. One type of offshore platform, a tension leg platform (“TLP”), is a vertically moored floating structure used for offshore oil and gas production. The TLP is permanently moored by groups of tethers, called a tension legs or tendons, which eliminate virtually all vertical motion of the TLP due to wind, waves, and currents. The tendons are maintained in tension at all times by ensuring net positive TLP buoyancy under all environmental conditions. The tendons stiffly restrain the TLP against vertical offset.
The offshore platforms typically support risers that extend from one or more wellheads or structures on the seabed to the platform on the sea surface. The risers connect the subsea well with the platform to protect the fluid integrity of the well and to provide a fluid conduit to and from the wellbore. During drilling operations, a drilling riser is used to maintain fluid integrity of the well. After drilling is completed, a production riser is installed.
As drilling rigs venture into ever increasing water depths and encounter new challenges, well control has become increasingly problematic. As costs of floating mobile offshore drilling units escalate, traditional time-intensive operations are constantly being re-evaluated in an effort to reduce overall non-drilling time, thereby increasing the drilling efficiency of the rig. With the economic pressures facing the oil industry today, it has become even more important to provide cost-effective alternatives to traditional drilling/well control methods.
Traditionally, offshore drilling is done either with a floating vessel, utilizing a subsea blowout preventer (BOP) stack, with full control and drilling riser systems or with a jackup or platform utilizing a surface BOP stack and controls. These methods could be viewed as safe and reliable, but not always the most cost effective. There are also concerns with other traditional control methods. For instance, another method utilizes a floating vessel with surface BOPs in place of subsea BOPs. High-pressure riser is run from the surface BOPs to the sea floor where it is cemented in place. This means that the rig is essentially cemented in place, allowing no practical means of disconnecting in the event of an emergency. Also, if anything damages the high-pressure riser while drilling, fluids in the riser escape to the environment.